1. Field of the Invention
The invention relates to an apparatus for polishing a substrate for planarization by chemical mechanical polishing. The invention relates further to a method of chemical mechanical polishing.
2. Description of the Related Art
FIGS. 1A to 1E illustrate respective steps in a method of forming a buried metal layer in a semiconductor device.
First, as illustrated in FIG. 1A, a semiconductor substrate 101 including active devices fabricated thereon is covered entirely with an insulating film 102.
Then, a resist film 105 having a certain pattern is formed on the insulating film 102, and subsequently, the insulating film 102 is etched with the patterned resist film 105 being used as a mask, to thereby form a contact hole 106 through the insulating film 102, as illustrated in FIG. 1B.
After removal of the resist film 105, as illustrated in FIG. 1C, a barrier film 103 composed of metal such as Ti or Ta is deposited over the insulating film 102 so that the contact hole 106 is covered at a sidewall and a bottom thereof with the barrier film 103.
Then, as illustrated in FIG. 1D, an electrically conductive layer 104 is deposited over the product to thereby fill the contact hole 106 with the electrically conductive layer 104.
Then, the electrically conductive film 104 is planarized by means of a chemical mechanical polishing apparatus 107, as illustrated in FIG. 1E. Thus, a buried metal layer 108 is formed.
The chemical mechanical polishing apparatus 107 includes a carrier on which a wafer to be polished is fixed, and a rotatable level block on which a polishing pad is mounted. A wafer is compressed onto a rotating polishing pad to thereby be polished. While a wafer is being polished by the polishing pad, polishing powder such as alumina or silica, and polishing slurry containing etchant such as H2O2 are supplied between the polishing pad and the wafer.
FIG. 2 illustrates a conventional apparatus for polishing a wafer by chemical mechanical polishing. The illustrated apparatus is comprised of a level block 23 connected to a rotatable shaft 24, a polishing pad 29 fixed onto the level block 23, a wafer holder 26 connected to a rotatable shaft 27 and holding a wafer 25 on a bottom thereof, and a slurry source 30 supplying polishing slurry onto the polishing pad 29 through a slurry supply port 21.
The wafer 25 is sandwiched between the polishing pad 29 and the wafer holder 26. While the wafer 25 is being polished by the polishing pad 29, polishing slurry 22 is supplied between the polishing pad 29 and the wafer 25 around a periphery of the wafer 25.
Though the illustrated apparatus is designed to have one wafer holder 26, the apparatus may be designed to have a plurality of wafer holders 26. For instance, the apparatus may be designed to have four wafer holders 26 equally spaced from one another above the level block 23 in order to concurrently polish four wafers at a time.
A conventional apparatus for polishing a wafer, such as the apparatus illustrated in FIG. 2, is accompanied with a problem of non-uniformity in polishing speed in a wafer, which results in that a wafer is polished around a center thereof to a greater degree than a periphery thereof.
In order to overcome this problem, there has been suggested a first polishing apparatus in which a polishing pad mounted on a level block is formed with a plurality of small through-holes through which polishing slurry is supplied onto a surface of the polishing pad from a polishing slurry source. The small through-holes are positioned in concentration with an axis of the polishing pad 29. Since polishing slurry is uniformly supplied between a wafer and the polishing pad, it would be possible to keep a polishing speed constant to thereby enhance uniformity in polishing a wafer.
There has been suggested also a second polishing apparatus in which a polishing pad is composed of porous material in order to enhance uniformity in polishing a wafer.
However, since a wafer having a greater diameter is compressed onto a polishing pad at a greater pressure around a center thereof than a periphery thereof, a polished wafer would have a cross-section like a cross-section of a concave lens, if a wafer is polished in accordance with the above-mentioned first or second polishing apparatuses in which polishing slurry is uniformly supplied to a surface of a wafer, whereas a polished wafer would have a cross-section like a convex lens, if a wafer is polished in accordance with the apparatus illustrated in FIG. 2.
In order to avoid this problem, Japanese Unexamined Patent Publication No. 5-13389 has suggested a polishing apparatus which has the same structure as that of the above-mentioned first and second polishing apparatus, but is capable of controlling an amount of polishing slurry at a predetermined position of a polishing pad for the purpose of enhancing uniformity in polishing a wafer.
Specifically, the suggested polishing apparatus is formed with a plurality of through-holes through which polishing slurry is supplied onto a surface of a polishing pad, in such a manner that the number of through-holes per a unit area in a region closer to a center of a polishing pad is designed to be greater than the number of through-holes per a unit area in a region closer to a periphery of a polishing pad, or that a through-hole located closer to a center of a polishing pad is designed to have a greater diameter than a diameter of a through-hole located closer to a periphery of a polishing pad.
A diameter of a wafer necessary to be polished is increasing. For instance, a diameter of a wafer to be polished years ago was 6 inches (about 15 cm), but a diameter of a wafer to be polished presently is in the range of 8 to 10 inches (about 20 to about 25 cm). Such a wafer having a great diameter could not be polished by means of such an apparatus as illustrated in FIG. 2, because the level block 23 has to have too much area, which results in too high load to the apparatus.
Hence, there has been suggested such a polishing apparatus as illustrated in FIG. 3A, in order to avoid the above-mentioned problem. The illustrated apparatus is comprised of a rotatable carrier 2 supporting a wafer 1 at a bottom thereof, a level block 3, a polishing pad 4 mounted on the level block 3 and positioned in facing relation to the carrier 2, and a motor 5 for rotating the level block 3 around a rotation axis. The polishing pad 4 is formed with a plurality of through-holes equally spaced from one another.
The wafer 2 is made to rotate, and then, is compressed onto the rotating polishing pad 4. Thus, the wafer 1 is polished. While the wafer 1 is being polished, slurry 6 is supplied onto a surface of the polishing pad 4 through the through-holes.
In order to enhance uniformity in polishing the wafer 1, the level block 3 is rotated by means of the motor 5 in such a manner that the rotation axis of the level block, 3 moves along an arcuate path. That is, the level block 3 makes so-called orbital revolution.
FIG. 4 shows a positional relation in orbital revolution between the wafer 1 rotating around a rotation axis A and the polishing pad 4 rotating around a rotation axis B. As illustrated in FIG. 4, if viewed from the rotation axis A, the rotation axis B rotates around the rotation axis A.
As mentioned earlier, if a wafer is polished with polishing slurry being supplied onto a surface of a polishing pad through through-holes formed with the polishing pad, there is caused a problem that a wafer is polished to a greater degree in a central region than in a peripheral region, resulting in that a wafer is concave in a central region thereof If a wafer is non-uniformly polished as mentioned above, an electrically conductive film such as the electrically conductive film 104 illustrated in FIG. 1D partially remains non-removed on an insulating film such as the insulating film 102, resulting in current leakage between wirings.
In order to avoid such a problem, it is necessary to sufficiently polish a wafer. However, this may result in that a wiring to be formed on an insulating film has different heights above a central region and a peripheral region of a wafer. Accordingly, a wiring resistance above a central region of a wafer becomes different from a wiring resistance above a peripheral region of a wafer with the result of deterioration in electro-migration (EM).